The invention relates to medical monitoring, and in particular, to pressure sensing by an implanted medical device.
There are many situations in which a patient requires long-term monitoring and when it may be desirable to implant a sensor for monitoring within the body of the patient. One such monitor is a pressure monitor, which can measure the pressure at a site in the body, such as a blood vessel or a chamber of the heart. When implanted in a vessel or a heart chamber, the sensor responds to changes in blood pressure at that site. Blood pressure is measured most conveniently in units of millimeters of mercury (mm Hg) (1 mm Hg=133 Pa).
The implanted pressure sensor is coupled to an implanted medical device, which receives analog signals from the sensor and processes the signals. Signals from the implanted pressure sensor may be affected by the ambient pressure surrounding the patient. If the patient is riding in an airplane or riding in an elevator in a tall building, for example, the ambient pressure around the patient may change. Changes in the ambient pressure affect the implanted pressure sensor, and may therefore affect the signals from the pressure sensor.
A typical implanted device that employs a pressure sensor is not concerned with total pressure, i.e., blood pressure plus ambient pressure. Rather, the device typically is designed to monitor blood pressure at the site of the internal sensor. To provide some compensation for changes in ambient pressure, some medical devices take additional pressure measurements with an external pressure sensor. The external pressure sensor, which may be mounted outside the patient""s body, responds to changes in ambient pressure, but not to changes in blood pressure. The blood pressure is a function of the difference between the signals from the internal and external pressure sensors.
Although the internal pressure sensor may generate analog pressure signals as a function of the pressure at the monitoring site, the pressure signals are typically converted to digital signals, i.e., a set of discrete binary values, for digital processing. An analog-to-digital (A/D) converter receives an analog signal, samples the analog signal, and converts each sample to a discrete binary value. In other words, the pressure sensor generates a pressure signal as a function of the pressure at the monitoring site, and the A/D converter maps the pressure signal to a binary value.
The A/D converter can generate a finite number of binary values. An 8-bit A/D converter, for example, can generate 256 discrete binary values. The maximum binary value corresponds to a maximum pressure signal, which in turn corresponds to a maximum pressure at the monitoring site. Similarly, the minimum binary value corresponds to a minimum pressure signal, which in turn corresponds to a minimum site pressure. Accordingly, there is a range of pressure signals, and therefore a range of site pressures, that can be accurately mapped to the binary values.
In a patient, the actual site pressures are not constrained to remain between the maximum and minimum monitoring site pressures. Due to ambient pressure changes or physiological factors, the pressure sensor may experience a site pressure that is xe2x80x9cout of range,xe2x80x9d i.e., greater than the maximum monitoring site pressure or less than the minimum monitoring site pressure. In response to an out-of-range pressure, the pressure sensor generates an analog signal that is greater than the maximum pressure signal or less than the minimum pressure signal. An out-of-range pressure cannot be mapped accurately to a binary value.
For example, the pressure sensor may experience a high pressure at the monitoring site that exceeds the maximum site pressure. In response, the pressure signal generates a pressure signal that exceeds the maximum pressure signal. The pressure signal is sampled and the data samples are supplied to the A/D converter. When the A/D converter receives a data sample that is greater than the maximum pressure signal, the A/D converter maps the data sample to a binary value that reflects the maximum pressure signal, rather than the true value of the data sample. In other words, the data sample is xe2x80x9cclippedxe2x80x9d to the maximum binary value. Similarly, when the A/D converter receives a data sample that is below the minimum pressure signal, the converter generates a binary value that reflects the minimum pressure signal rather than the true value of the data sample.
Because of changes in ambient pressure, pressures sensed by the internal pressure sensor may be in range at one time and move out of range at another time. When the pressures move out of range, some data associated with the measured pressures may be clipped, and some data reflecting the true site pressures may be lost. In such a case, the binary values may not accurately reflect the true blood pressures at the monitoring site.
To avoid clipping, the implanted device may be programmed to accommodate an expected range of site pressures. Estimating the expected range of site pressures is difficult, however, because ambient pressure may depend upon factors such as the weather, the patient""s altitude and the patient""s travel habits. Pressures may be in range when the patient is in one environment, and out of range when the patient is in another environment.
The risk of clipping can further be reduced by programming the implanted device with a high maximum site pressure that corresponds to the maximum binary value and with a low minimum site pressure that corresponds to the minimum binary value. Programming the device for a high maximum and a low minimum creates a safety margin. The price of safety margins, however, is a loss of sensitivity. Safety margins mean that pressures near the maximum and minimum site pressures are less likely to be encountered. As a result, many of the largest and smallest binary values are less likely to be used, and the digital data is a less precise representation of the site pressures.
In general, the invention is directed to a pressure monitor for use in an implantable medical device that uses automatic pressure range adjustment to keep data samples within the range of an A/D converter, and thereby avoid clipping. In addition, the invention automatically adjusts pressure range to preserve high sensitivity.
An A/D converter maps signals from a pressure sensor to binary values, which are supplied to a controller such as a microprocessor. The controller generates a histogram of the digital pressure data from the binary values. The histogram reflects the distribution of the digital pressure data, which in turn should reflect the distribution of pressures at the monitoring site. The histogram is divided into a number of xe2x80x9cbinsxe2x80x9d that correspond to a set of pressure values. The contents of the bins are a function of the pressures at the monitoring site. The contents of the bins are also a function of the parameters that define how site pressures are mapped to binary values. The distribution of digital pressure data in the bins therefore not only provides useful information about the pressures at the monitoring site, but also information as to whether there is a risk of data going out of range. The distribution of pressure values in the bins further provides information as to whether there is xe2x80x9cunused range,xe2x80x9d i.e., a range of binary values that was not used.
The controller senses the possibility of out-of-range data or unused range by sensing the contents of the lowest and highest bins of the histogram. If the lowest bin is full, for example, that may indicate that data are out of range on the low side. If there are several bins on the low side that are empty, however, that may indicate that range on the low side is not being efficiently utilized. Similar conditions are checked on the high end of the histogram. When it appears that data may be out of range or that range is not being used, the controller adjusts the mapping parameters, with the goal of generating a new histogram that does not have the same problems.
One way in which the controller may adjust the histogram parameters is by controlling the gain and the offset of an amplifier. Adjusting the gain expands or decreases the range of pressure signals that are supplied to the A/D converter. Adjusting the offset moves the range of pressure signals up or down.
In one embodiment, the invention is directed to a method comprising mapping a set of blood pressures to a set of discrete binary values with first mapping parameters. The first mapping parameters include a maximum binary value that corresponds to a maximum blood pressure and a minimum binary value that corresponds to a minimum blood pressure. The method also includes generating a histogram with the set of discrete binary values and generating second mapping parameters as a function of the contents of the lowest bin and the highest bin of the histogram. The second mapping parameters may be generated by adjusting the gain and/or offset of an amplifier. The method may further include mapping a second set of blood pressures to a second set of discrete binary values with the second mapping parameters and generating a second histogram with the second set of discrete binary values.
In another embodiment, the invention is directed to a computer-readable medium containing instructions that cause a programmable processor to carry out this method.
In further embodiment, the invention is directed to a device comprising an amplifier, an analog-to-digital converter and a controller. The amplifier generates pressure signals as a function of pressure sensed by a pressure sensor in a body and as a function of mapping parameters, and the analog-to-digital converter that converts the pressure signals to a set of discrete binary values. The controller generates a first histogram as a function of a distribution of a first set of binary values. The controller also generates second mapping parameters as a function of the distribution of data in the first histogram.
In an additional embodiment, the invention presents a method comprising receiving analog pressure data, mapping the analog pressure data to discrete binary values with first mapping parameters, generating a first histogram as a function of the distribution of the discrete binary values during a storage interval and, after the storage interval, generating second mapping parameters as a function of the contents of the lowest bin and the highest bin of the histogram.
The invention can provide a number of advantages. For example, the invention automatically reduces the risk that data may go out of range and be clipped, by automatically adjusting the range of the data, thereby automatically correcting for changes in the ambient air pressure experienced by the patient. The patient""s physician need not program the device with an expected maximum and minimum pressure values. In addition, the invention automatically keeps the data in range with little adverse effect on sensitivity. The invention also increases sensitivity when unused range is detected.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.